1. Field of the Invention
The present invention relates generally to the field of phytoremediation. More particularly, the invention relates to methods of removing and possibly recovering a variety of contaminants such as, for example, metals, including heavy metals from soil using creosote (Larrea tridentata) plants.
2. Description of Related Art
Numerous ecological and health problems are associated with environmental contamination. Heavy metal contamination of soil is a particular hazard, due to the almost indefinite environmental persistence of metals, as well as the universal toxicity of high concentrations of many heavy metals in living organisms. The term "heavy metals" as used herein, means any metal having a molecular weight greater than sodium. Additionally, sometimes various elements such as selenium and arsenic are considered to be in this category. While some heavy metals are naturally present in soil, heavy metal contamination is also often the result of human activities. For example, mining, smelting, intensive agriculture, sludge dumping, energy and fuel production, electroplating and fuel exhaust all contribute to contaminating the soil with heavy metals. Currently, the most common methods for dealing with this problem remain removal and burial of the contaminated soil or the isolation of the contaminated area. Removal and burial is extremely expensive. Between 1995 and 1999 the United States is expected to spend seven billion dollars cleaning up sites contaminated with heavy metals alone and another thirty-five billion dollars cleaning up sites contaminated with both metals and additional organic contaminants (Brown). Cleaning up a single acre of contaminated land can cost as much as one million dollars (Powell).
One relatively new option for decontaminating soil is phytoremediation or phytoextraction, the use of plants to scavenge organic or inorganic contaminants from the soil (Baker et al., 1989, 1994; Chaney; Cunningham and Berti; Raskin et al, 1994; Wenzel et al). While most plants are unable to grow in soil containing high levels of heavy metals, a few species appear to be able to absorb the metals, adapt to them and thrive. Such plants are sometimes called hyperaccumulators or metallophytes.
Certain plants have been found that accumulate nickel (Brooks et al., 1979), cobalt and copper (Brooks et al, 1978), manganese (Brooks et al., 1981), lead and zinc (Reeves and Brooks,), zinc and cadmium (Brown et al., 1994), and selenium (Banuelos and Meek). Accumulators of nickel are by far the most common amongst the hyperaccumulators discovered to date. Of the 400 known phytoremediator plants known in 1995, 300 absorb nickel, while about 26 assimilate cobalt, 20 take up copper and another 20 incorporate zinc (Brown). The more rarely absorbed heavy metals include manganese, cadmium and lead, with lead being particularly hard to absorb, as well as being very difficult to translocate from plant roots into plant stems after it is absorbed (Kabata-Pendias and Pendias; Reeves and Brooks; Malone et al.; Zimdahl and Koeppe; and Koeppe). One recently reported lead accumulator is Brassica juncea, the Indian mustard plant (Kumar et al., 1995). Some cultivars of this species are particularly efficient at translocating lead into their stems, although lead levels in the leaves was still low, 11% to 5.6% of the amount found in the stems.
Despite ever increasing interest and research in the field, several problems associated with phytoremediation remain (Brown). For example, some metals in contaminated areas may be unreachable via phytoremediation because they lie beneath the plant root zone. Many metal-accumulating plants are unable to translocate a substantial proportion of the metal they acquire from their roots to other tissues, and many of the known metal-accumulating plants are simply too small to accumulate large quantities of metal. Additionally, many of the plants thus far identified as useful in phytoremediation are from tropical regions (Moffat). For the future usefulness of phytoremediation, plants are needed that can grow in a variety of climates, from deserts to the coldest locations with human activity.